1. Field of the Invention
This invention relates to a method and apparatus for measuring the irregularities in the shape of a circumferentially deformable cylindrical element, such as a large diameter plastic pipe. More particularly, this invention relates to a method and apparatus for measuring the ovality of plastic pipe. In addition, this invention relates to a method and apparatus for determining the scratch depth of a scratch on the outer surface of the plastic pipe.
2. Description of Prior Art
When large diameter plastic pipe is wound into a coil, the circular cross-section of the pipe is distorted into a roughly elliptical shape. When large diameter coiled pipe is then unwound for installation, for example in connection with utility operations such as natural gas transmission and distribution, a re-rounding mechanism is used to restore the circular cross-section of the plastic pipe. Due to the visco-elastic properties of the plastic pipe, the re-rounding mechanism distorts the elliptical cross-section of the pipe into an elliptical cross-section that is perpendicular to the major axis of the original elliptical cross-section. It is, thus, apparent that the ability to determine the longest (major) axis of an out-of-round pipe is an important aspect of correcting the out-of-roundness. After re-rounding, the pipe creeps into a nearly circular shape. To achieve a circular cross-section after re-rounding, the re-rounding mechanism must be adjusted to account for the size, cross-sectional shape and composition of the pipe, temperature of the pipe, and rate of travel of the pipe through the re-rounding mechanism. In current practice, a hand micrometer or caliper is used to measure the major and minor diameter of the pipe as it leaves the re-rounding mechanism and before the pipe reaches the ground behind a coiled pipe trailer.
Measuring ovality of the pipe is particularly critical for joining ends of the pipe together. For example, in preparing polyethylene pipe sections for joining, it is critical that the cross sectional profile of the ends are congruent in order for a proper fusion joint to occur. In addition, for the proper application of saddles, tees and other fittings, the circumferential shape of the pipe must be such that the pipe wall is in large part coincident with the mating surface of the applied device, again in order to insure a proper fusion joint between the two components.
In addition to ovality, measuring scratch depth is critical for determining the suitability of a section of plastic pipe. Manufacturers' guidelines frequently specify a maximum acceptable depth of a scratch as 10% of the nominal wall thickness; deeper scratches usually require the removal of the involved section due to the potential weakening of the pipe from the scratch.
There exists several known mechanisms in the prior art for profiling and measuring the surfaces of cylindrical objects, some of which are used in conjunction with manufacturing processes for controlling the roundness of the objects produced. For example, U.S. Pat. No. 5,345,309 to Wertz et al. teaches a system for generating a solid model of the sidewall of a container in which a mandrel selectively holds the container being evaluated in two locations. In the first location, a non-contact measurement apparatus utilizing a digital camera produces digital information for generating a three-dimensional sidewall surface profile of the container based upon shadow edge detection. In the second position, a contact measurement apparatus produces information for generating a three-dimensional sidewall thickness map of the container. The computer collects the information from the measurement apparatuses and produces an overall solid model of the sidewall of the container based upon sidewall surface profile and sidewall thickness. The computer generates object tilts, diameter, ovality, and squarity information. The information thus collected is then used to control the manufacture of additional cylindrical objects. The contact measurement apparatus includes a plurality of LVDTs (linear variable differential transformers), each of which interacts with a mechanical contact apparatus to mechanically contact various locations on the sidewall of the object to provide sidewall thickness measurements at a plurality of circumferential locations on the object. The contact apparatuses are mechanically in contact with the linear variable differential transformers which, in turn, are electrically connected to a linear variable differential transformer interface which then delivers the contact measurements to a multiplexer. The multiplexer interconnects with an analog to digital converter which, in turn, delivers digital data to a computer. Each contact apparatus along the sidewall of the object adjusts to a desired elevation under control of a stepper motor, becomes activated to perform measurements under control of an actuator, and engages its own linear variable differential transformer to deliver the measurements.
U.S. Pat. No. 4,700,484 to Frank et al. teaches an apparatus for measuring the diameter of an object in which a rotatable wheel of known diameter capable of movement in three axes is contacted with an object capable of rotation. The wheel is attached to a shaft encoder which produces pulses as the wheel rotates. As the object is rotated, start and end reference marks are sent and the pulses produced by the shaft encoder are counted. A microprocessor calculates the diameter of the object based upon the wheel diameter, counts per revolution of the wheel, and the counts per revolution of the object.
U.S. Pat. No. 4,240,206 to Baresh et al. teaches an ovality measuring device and method for measuring the ovality of tubing comprising an anvil having an electromechanical probe which measures across a tube while a second anvil and electromechanical probe measure across the tube at right angles to the first anvil and probe. Signals from each of the probes are summed and displayed to show the ovality of the tubing. The anvils and probes are mounted to a head that is rotated open to permit placement over tubing and resiliently rotated closed to bring the probes in contact with the tubing.
One of the requirements of known apparatuses for measuring the ovality of cylindrical objects is access to the entire cylindrical object circumference. However, there are many instances in the utility industry, in particular, in the case of partially buried pipelines, where access to the entire circumference of the pipe at a given location along the length of the pipe is not available. Thus, there is a need for a method and apparatus for determining the ovality of a cylindrical object which does not require access to the entire circumference of the cylindrical object in order to make an accurate determination of the ovality of the cylindrical object.